1. Field of the Invention
The present invention relates to a method of an apparatus for compressing and decompressing image data.
2. Description of the Related Art
FIG. 31 is a block diagram showing a structure of conventional image data compression apparatus and a decompression apparatus. In a image data compression apparatus 540, original image data is orthogonally transformed with respect to every pixel block of M.times.N pixels, where M and N are integers, by an orthogonal transform unit 542, quantized by a quantization unit 544, and finally coded by an entropy coding unit 546 to produce compressed image data. In a image data decompression apparatus 550, the compressed image data is decoded by an entropy decoding unit 556, inversely quantized by an inverse quantization unit 554, and then inverse-transformed by an inverse orthogonal transform unit 552 to produce reconstructed image data. The quantization unit 544 and the inverse quantization unit 554 utilize an identical quantization table 562, and the entropy coding unit 546 and the entropy decoding unit 556 use an identical code table 564.
It sometimes happens that an image to be compressed includes a first image area which is to be reconstructed with a high image quality and a second image area which may be reconstructed with a lower image quality. In such a case, image data of the first image area is quantized with a quantization table having relatively low quantization levels while image data of the second image area is quantized with another quantization table having relatively high quantization levels. A quantization table is a matrix having the same size as a pixel block, that is, an M.times.N matrix. In the conventional image data compression/decompression apparatus, when a plurality of quantization tables are needed to one image, the plurality of quantization tables each consisting of an M.times.N matrix should be transferred from the compression apparatus 540 to the decompression apparatus 550. This undesirably increases the size of compressed video data.
Incidentally, when an image to be compressed includes a simple and monotonous image area consisting of a series of pixel blocks uniformly painted with a predetermined color, M.times.N matrices of orthogonal transform coefficients corresponding to the plurality of pixel blocks are all entropy coded according to the conventional apparatus. Compressed image data representing such a simple and monotonous image area therefore requires a relatively large data size.